1. Field of the Invention
The present invention relates to an image display apparatus that presents a virtual image of an image displayed on a display element to an observer, and also relates to a head-mounted display (hereinafter also referred to as “HMD”) that is provided with such an image display apparatus.
2. Description of Related Art
There have conventionally been proposed various image display apparatuses that allow observation of a virtual image of an image displayed on a display element such as a liquid crystal display element (LCD). For example, Patent Document 1 listed below proposes an image display apparatus that employs as a light source an LED that emits red (R), green (G), and blue (B) light wherein a reflective LCD is irradiated with the R, G, and B light on a time-division basis and the light exiting from the LCD is directed through a main prism portion to an observer's eye.
For another example, Patent Document 2 listed below proposes an image display apparatus wherein a reflective LCD is illuminated by an LED that emits R, G, and B light and the light exiting from the LCD is reflected on a Lippmann three-dimensional hologram and is thereby directed to an observer's eye.
On the other hand, apparatuses called HMDs have been generally known that are mounted on an observer's head and that project a virtual image of an image produced by an image display element through an eyepiece optical system to the observer's eye. These HMDs have, housed in their casing, the image display element along with a light source for illuminating it. In recent years, as increasingly compact, lightweight HMDs are developed, light-emitting diodes (hereinafter also referred to as “LEDs”) have come to be generally used as such light sources for their compactness and low cost.
Inconveniently, as an HMD is used for a long period, the rise in temperature inside its casing resulting from the heat generated by an LED may cause undesirable conditions such as a drop in image display brightness and deterioration of components (for example, a heat-induced deformation in a component). To avoid these inconveniences, for example, Patent Documents 3 and 4 listed below propose a method of alleviating the rise in temperature inside an image display apparatus by use of a cooling fan provided inside it. Providing a head-mounted apparatus with a cooling fan, however, makes it large and heavy, making it difficult for an observer wearing the apparatus on his head to use it for a long period.
For another example, Patent Document 5 listed below proposes a method of letting the heat generated by an LED escape out of an endoscope. Specifically, on a circuit board arranged at the tip of the inserted portion of an endoscope, a signal conductor pattern that is electrically connected to an LED is arranged parallel to a ground conductor pattern with an insulating region lying in between. Here, the insulating region is a region where no conductive pattern is arranged, and is formed to surround the periphery of the signal conductor pattern. The ground conductor pattern is grounded via a shield portion of the cable across which electric current is supplied to the LED. In this structure, the heat generated by the LED travels from the signal conductor pattern via the insulating region to the ground conductor pattern, and is then expelled via the shield portion of the cable.
Here is a list of the patent documents mentioned above:
Patent Document 1: U.S. Pat. No. 6,680,802 B1
Patent Document 2: JP-A-H11-326821
Patent Document 3: JP-A-H06-175066
Patent Document 4: JP-A-H09-034376
Patent Document 5: JP-A-2003-024276
In an image display apparatus in which an LCD is driven on a time-division basis, the LCD is irradiated with R, G, and B light on a time-division basis. This permits the light source itself to accumulate a comparatively small amount of heat, and thus the wavelength of the light emitted from the light source is less prone to vary.
In contrast, in an image display apparatus that employs an LCD having, for example, RGB color filters, since the color filters have low light transmissivity, in order to present a bright image, it is necessary to use a high-brightness light source, that is, a light source that emits light with high light intensity. When such a high-brightness light source is used, however, the light source itself accumulates heat, and this causes the wavelength of the emitted light to vary.
On the other hand, a volume-phase-type reflective hologram optical element such as a Lippmann three-dimensional hologram has wavelength dependence, meaning that it only diffracts (reflects) light of a particular wavelength incident on it at a particular angle of incidence. Thus, if, as a result of a light source accumulating heat, the wavelength of the light emitted from it varies and consequently the peak-intensity wavelength of the light emitted from the light source deviates from the peak-diffraction-efficiency wavelength of the hologram optical element, then the light emitted from the light source cannot be diffracted with the hologram optical element efficiently.
Accordingly, in an image display apparatus employing a volume-phase-type reflective hologram optical element, when a high-brightness light source is used in order to present a bright image to the observer, a variation in the wavelength of the light emitted from the light source dims the image presented through the hologram optical element. Moreover, a variation in the wavelength of the light emitted from the light source also degrades the color reproduction of the image presented to the observer.
What is to be noted with Patent Document 5 is that it simply discloses a technique for heat rejection from an LED in an endoscope and that the technique is not intended for application in image display apparatuses.